The application of protective coatings of polymers to electrically conductive surfaces by the electrodeposition thereon of organic polymeric materials is of established industrial importance. In recent years it has been shown that many of the shortcomings of the earlier anodic electro-deposition processes are obviated when the metallic article to be coated is used as the cathode rather than as the anode, the article to be coated being immersed in an aqueous bath containing an ionized derivative of the polymeric material to be deposited.
In order to be commercially acceptable, a polymeric composition for use in cathodic electro-deposition processes must be capable of forming a stable aqueous solution or dispersion which possesses sufficient concentration with respect to the polymer and which exhibits sufficient electrical conductivity to be capable of depositing a film of polymeric material on a metallic substrate, when an electric current is passed through the aqueous solution or dispersion, using the substrate as a cathode. Furthermore, the electro-deposited polymeric composition must have good film-forming properties and must not tend to run off, or to sag on, vertical surfaces when the object is removed from the electro-deposition bath, and to be displaced when the object is rinsed before further steps in the coating process. The deposited film must display good adhesion to the metallic substrate before drying, after drying and after any cross-linking of the deposited film. The dried and cross-linked coating should desirably be unstained, durable and corrosion resistant and should exhibit good adhesion to any subsequent coatings.
A further requisite of the aqueous polymeric solution or dispersion is the property of good throwing power, i.e., the ability of the solution or dispersion to deposit on the substrate a coating of uniform thickness, even on those areas of the substrate which are remote from or physically shielded from a counter-electrode. For many purposes, for example, for primer coatings for automobile bodies, a high degree of toughness is essential, combined with a degree of flexibility.
Certain low molecular weight so-called epoxy resins well known in the prior art, for example, the reaction products of epichlorhydrin with alpha, omega-dihydroxy compounds, or the low molecular weight polymers and copolymers derived from epoxy-alkyl acrylates or methacrylates, after modification by reaction with certain boron esters or with amines, are known to exhibit many of the aforesaid desirable properties required of cathodically electrodepositable polymeric compositions. It may happen that the necessary degree of durability or scratch resistance, after cross-linking in order to form the protective coating, cannot be achieved with the accompaniment of a sufficient degree of flexibility and/or corrosion resistance. It is also necessary that such a product, after modification and admixture with water in order to form an aqueous solution or dispersion, should contain free epoxide or other reactive functional groups which are capable of participation in cross-linking reactions during the coating process at a stage subsequent to electro-deposition. These reactive groups required for cross-linking, or their precursors, are introduced during the first stage of synthesis of the low molecular weight polymer and are very reactive. The objective of so modifying the polymer in order to promote water-solubility such that it is cathodically electro-depositable, and additionally to provide for the presence of reactive functional groups for the sake of cross-linking after the step of electro-deposition on the metal substrates, is difficult to achieve without at least incurring the risk of cross-linking the polymer at some stage before electro-deposition. Such a result is highly undesirable in that premature gelation may occur, leading to a product difficult or impossible to dissolve or disperse in water.
It may also be necessary to provide one or more specific agents which are co-reactive with the reactive groups of the low molecular weight resin, in order to provide for and/or in order to promote cross-linking, and to achieve an acceptable combination of properties of the final protective coating. These additional agents must be introduced into and dispersed in the aqueous composition.
A further drawback attached to use of such prior art compositions is that the starting materials needed for synthesis of the low molecular resins are often relatively costly.
The prior art describes processes for coating electrically conductive articles whereby the article is used as a cathode in an aqueous bath containing a basic resin which is at least partially neutralised with an acidic substance such as an organic carboxylic acid, wherein it is suggested that the basic resin may be derived, inter alia, from epoxidised polybutadiene. However, it is not always specified that the epoxidised polybutadiene should be a liquid substance, and where it is suggested that low molecular weight polybutadienes are used which are probably liquid polybutadienes, the recommended molecular weight of the polymer is so low that difficulties would be liable to occur during the electro-deposition, such as inefficient throwing power and/or run down of the coating on vertical surfaces. Furthermore, the conditions for conversion of epoxidised polybutadiene into basic cathodically electro-depositable resins are seldom specified and as will be shown hereinafter, the possibilities exist on the one hand of inadequate reaction of the polybutadiene epoxide with organic amines in order efficiently to form cathodically electro-depositable resins, or on the other hand, of giving rise to gelation during said reaction. (U.S. Pat. No. 3,716,402 and British Pat. No. 1,407,410).